After 2 years of trying to squat to parallel with good form, I finally found that nothing loosened my hips like pause squats. Squat down with a light weight (I used 135-155 when I was squatting 225) to parallel or as low as you can with a medium stance (with a straight back), pause for a 2 count, then back up.
His rationale is the following:
This makes a lot of sense when you realize that it is a form of proprioceptive neurofacilitation (PNF) stretching. You are strengthening the muscles the in stretched positon while fatiguing them and forcing them to stretch further. Since flexibility is directly related to strength in a given ROM, this works better than regular stretching.